Verifier question

Question

I'm trying to figure out a detail of the verification algorithm. I created the following method by hand:
public static void main(java.lang.String[]);
Signature: ([Ljava/lang/String;)V
Code:
0: iconst_0
1: istore_1
2: aconst_null
3: astore_1
4: nop
5: goto 11
8: astore_2
9: iload_1
10: pop
11: return
Exception table:
from to target type
2 4 8 Class java/io/IOException
This seems to me like it would have a local variable conflict in register 1 at instruction 9, but it verifies and runs fine. If I change the exception handler so that the end_pc is 5 (so that it includes the NOP), the method does not verify and tells me that register 1 contains wrong type. Similar things happen if I use anything in place of the NOP... the verifier just doesn't like having anything in the try{} block after that ASTORE. Can anyone tell me why it is ok with it when the ASTORE is the last instruction?? Thanks.
--Mike Stepp, University of Arizona
([email protected])

Answer 1

wow... pretty complicated stuff :p Its like
programming in assembly. Are u using BCEL or
JavaAssist?
If u r like me, pretty bad in assembly and VM/bytecode
internals, I would write the code that I want and use
javap -c to figure it out. Sorta of "reverse
engineer" :)Unfortunately, I can't just make javac produce the code I want...
I'm working for a project that performs Java bytecode obfuscation/analysis/watermarking, so I need to know exactly what is legal code, not just the code that comes out of javac (or even jikes for that matter).
I have been using BCEL, which is a really nice system.
Anyway, thanks for taking an interest. :-)